I was prone to this when I first started out. Mine was a lack of timing (starting my forward cast before my backcast was completely finished) and trying to hard to get a good cast (using too much muscle on the forward cast). So, I agree with namekagon and n2permit. I fixed this problem by turning my head and watching my backcast. It sounds simple and therein lies the beauty of it, it is simple. Keep practicing and it will become simple to you. Good luck.